1. Field of the Invention
The present invention generally relates to the protection of switches. More specifically, the present invention relates to the protection of a bidirectional switch upon occurrence of an overcurrent, resulting for example from a short-circuit in a load controlled in A.C. mode by the switch.
2. Discussion of the Related Art
FIGS. 1A and 1B illustrate a known method for protecting a bidirectional switch for controlling an A.C. load against overcurrents.
FIG. 1A schematically and partially illustrates a load 1 series-connected with a bidirectional switch 2 across an A.C. voltage source 3. Bidirectional switch 2 is formed by the anti-series connection, between two terminals A and C, of two insulated-gate bipolar transistors (IGBT) 4 and 5. “Anti-series” means that IGBT transistors 4 and 5 are in series and that their junction point is a common emitter point. Each terminal A and C is then connected to the collector of one of transistors 4 and 5. For clarity, the collector of transistor 4 is designated in the following description of FIG. 1 as “A” and the collector of transistor 5 is designated as “C”. The emitter of each of transistors 4 and 5 is connected to its respective collector A, C by a respective diode D1, D2. Each diode D1, D2 is in antiparallel with the junction of its associated transistor 4, 5. Collector A of transistor 4 is connected to a supply terminal of load 1. Collector C of transistor 5 is connected to a terminal of source 3, non-connected to load 1. A circuit 6 for controlling and protecting transistor 4 is connected between its gate G1 and its collector A. Similarly, for transistor 5, another control and protection circuit 7 is connected between its gate G2 and its collector C. The two protection circuits 6 and 7 are identical.
FIG. 1B schematically and partially illustrates a protection circuit (6 or 7, FIG. 1A) connected between a collector A or C and a gate G1 or G2. A control block 10 (CTRL) comprises two supply terminals respectively connected to a high supply rail Vcc and a low supply or ground rail GND connected to the emitter of protected transistor 4 or 5. An output terminal of block 10 is connected to an end of a resistor 11 having another end forming the output terminal of protection circuit 6, respectively 7, connected to gate G1, respectively, G2, of transistor 4, respectively 5. An input terminal of block 10 is connected to an output terminal OUT of a comparator 12. Two supply terminals of comparator 12 are respectively connected to high supply rail Vcc and low supply rail GND. An inverting input IN1 of comparator 12 is connected to a reference D.C. voltage source (V) 13. A non-inverting input IN2 of comparator 12 is connected to high supply rail Vcc, via a biasing resistor 14. Non-inverting input IN2 is also connected to the anode of a diode 15 having its cathode connected to collector A, respectively C, of protected unidirectional switch 4, respectively 5. Circuits 6, respectively 7, ensure their protection function by controlling gate G1, respectively G2, of transistor 4, respectively 5, according to the result of the comparison, by comparator 12, of the current value of the collector-emitter voltage with voltage reference V provided by source 13.
The protection circuit of FIG. 1B enables controlling collector-emitter voltage Vce across protected transistor 4 or 5 by means of comparator 12. Given the current-vs.-voltage characteristic of a transistor, a voltage Vce unusually high as compared to the reference set by source 13 corresponds to the occurrence of an overcurrent, linked to a malfunction of load 1 or of source 3. Diode 15 is a protection diode intended to protect non-inverting (+) input IN2 of comparator 12, especially when protected transistor 4 or 5 is off.
A disadvantage of the structure previously described in relation with FIGS. 1A and 1B is the need to repeat twice a protection circuit of a one-way switch to obtain a bidirectional switch protection circuit.
Another disadvantage of the previous structure is that protection diodes 15 of circuits 6, 7 must be able to hold a high voltage, especially when switch 2 is off. High-voltage diodes are relatively complex and bulky to make in integrated form.
It has previously been considered that bidirectional switch 2 is formed of the anti-series connection of two IGBT transistors, each being associated with a free wheel diode in anti-parallel. However, the same disadvantages are encountered if the transistors are of MOS type.
FIG. 2 schematically and partially illustrates another known embodiment in which bidirectional switch 2 is formed of the antiparallel connection of two IGBT or MOS transistors of same conduction type T1 and T2, each transistor T1, T2 being in series with a respective rectifying diode D3, D4. For clarity, the connection of switch 2 in series with load 1 across A.C. source 3 described in relation with FIG. 1A is indicated only by the mentioning of terminals A and C in FIG. 2. Terminal A is connected to the anode of diode D3, the cathode of which is connected to the collector of transistor T1. Terminal A is also connected to the cathode of diode D4, the anode of which is connected to the emitter of transistor T2. Terminal C is connected to the emitter of transistor T1 and to the collector of transistor T2.
Protection circuit 17 of switch 2 here is comprised of two separate comparators 121 and 122. Non-inverting (+) input IN21 of comparator 121 is connected to the anode of a diode 151 having its cathode connected to the collector of transistor T1 (cathode of diode D3). The inverting (−) input IN11 of comparator 121 receives a reference voltage Vref+, positive with respect to the ground defined by one of the two terminals of switch 2, for example, terminal C, and provided by a voltage source 131.
Non-inverting (+) input IN22 of comparator 122 is connected to the cathode of a diode 152 having its anode connected to the emitter of transistor T2 (anode of diode D4). Inverting (−) input IN12 of comparator 121 receives a reference voltage Vref−, negative with respect to ground GND and provided by a second voltage source 132.
The respective outputs OUT1 and OUT2 of comparators 121, 122 are connected to input terminals of a control circuit (not shown) driving, generally via resistors (not shown), gates G1 and G2 of transistors T1 and T2.
The supply of comparator 121 is ensured by a source 133 of a positive supply voltage +Vcc connected between a supply terminal of comparator 121 and ground GND. Similarly, a source 134 of a negative supply voltage −Vcc is connected between a supply terminal of comparator 122 and ground GND.
The operating principle of protection circuit 17 of FIG. 2 is similar to that of a protection circuit 6, 7 of FIGS. 1A and 1B, voltage Vce of each transistor being compared with a respective reference Vref+, Vref− set by respective source 131 or 132. Diode 151, 152 of each portion of circuit 17 dedicated to the protection of one of the two switches unidirectional in current T1, D3 and T2, D4 is homologous to diode 15 of each circuit 6, 7 of FIG. 1.
A disadvantage of such a structure is the need to provide two voltage reference supply sources 133 and 134.
Another disadvantage of such a structure is the presence of high-voltage diodes 151 and 152.
According to another known method, a read resistor is introduced in series with the load and the bidirectional switch and the occurrence of overcurrents across this resistor is detected. As compared to the diagram of FIG. 2, the two non-inverting inputs of comparators 121 and 122 are then connected to the junction point of the switch and the detection resistor, the other terminal of this resistor being connected to ground, which corresponds to one of the terminals of application of the A.C. supply voltage. Diodes 151 and 152 are then no longer necessary.